This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2002-097913, filed Mar. 29, 2002; No. 2002-146561, filed May 21, 2002; and No. 2002-146562, filed May 21, 2002, the entire contents of all of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an ink for forming a hole injection layer of an organic EL (electro luminescence) display device, which makes it possible to manufacture an organic EL display device excellent in luminance and in long life, and to the method of preparing such an ink. The present invention also relates to a method of manufacturing an organic EL display device by using the aforementioned ink, and to the organic EL display device thus manufactured.
2. Description of the Related Art
Recently, an electro luminescence (EL) element where a multi-layer film formed of an organic material is employed is attracting many attentions (for example, Japanese Patent Unexamined Publications (Kokai) S63-264692; S63-295695; H1-243393; and H1-245087). This organic EL element has been conventionally manufactured mainly by two kinds of manufacturing methods, i.e. a manufacturing method using the vacuum deposition of a low molecular weight material, and a manufacturing method using the coating of a polymer solution. Among them, the latter method is advantageous in that it is suited for easily forming a large display area and, in particular, suited for manufacturing a full color display with high resolution and large size of the picture plane by using an ink jet process.
In a polymer organic EL element, a hole injection layer is usually provided in order to decrease the voltage to be applied, to enhance the emission efficiency and to increase the useful life thereof. This hole injection layer can be formed by coating an ink comprising an associated body containing a donor molecule and an acceptor molecule, which is dispersed in water. However, there is a problem that since this hole injection layer contains a low molecular weight solid component, this solid component is permitted to reach a polymeric luminescence layer due to the diffusion of the low molecular weight solid component as the EL element is actuated for a long period of time, thereby enabling the low molecular weight solid component to function as a trapping agent or a quencher, thus leading to the deterioration of the display characteristics.
Furthermore, since the ink containing this low molecular weight solid component is liable to flocculate, there are raised various problems such as the deterioration in reproducibility of the coating for forming the film by an ink jet method thus giving rise to the generation of malfunction of display, the deterioration in flatness of the film to be obtained, or the deterioration in properties and life of the organic EL.
Additionally, if this hole injection layer is inappropriately formed, the hole injection efficiency and the hole-transporting property thereof would be deteriorated, thus deviating the carrier balance between the electrons and the holes, thus giving rise to the deterioration in life and emission efficiency of display device.
Incidentally, this organic EL (electro luminescence) element is also referred to as an organic light emitting diode element or as an organic light emitting diode. On the other hand, the organic EL display device is also referred to as an organic light emitting diode display device, as an organic light emitting display, or as an organic light emitting device. Further, the term xe2x80x9celectro luminescencexe2x80x9d is frequently referred to simply as xe2x80x9cELxe2x80x9d.
According to one aspect of the present invention, there is provided an ink for forming a hole injection layer of organic electro luminescence display device, the ink comprising:
water; and
solid matters dispersed in the water, the solid matters containing a donor molecule comprising polythiophene and/or a derivative thereof, and an acceptor molecule comprising polystyrenesulfonic acid and/or a derivative thereof;
wherein sulfate ion is limited to 1 ppm, and components having a molecular weight of 110,000 or less is limited to 35 wt % based on an entire weight of the solid matters, the molecular weight being converted as sodium polystyrenesulfonate in an aqueous gel permeation chromatography measured by using UV of 254 nm in wavelength.
According to another aspect of the present invention, there is provided a method for preparing the ink according to one aspect of the present invention, the method comprising:
preparing a dispersion liquid comprising water, and solid matters dispersed in the water, the solid matters containing a donor molecule comprising polythiophene and/or a derivative thereof, and an acceptor molecule comprising polystyrenesulfonic acid and/or a derivative thereof; and
subjecting the dispersion liquid to at least one of dialysis and ultrafiltration to limit the concentration of sulfate ion in the ink to 1 ppm or less and to limit the ratio of components having a molecular weight of 110,000 or less to 35 wt % or less based on an entire weight of the solid matters, the molecular weight being converted as sodium polystyrenesulfonate in an aqueous gel permeation chromatography measured by using UV of 254 nm in wavelength.
According to a further aspect of the present invention, there is provided a method for manufacturing an organic electro luminescence display device provided with a two-dimensional array of pixels comprising organic electro luminescence elements capable of emitting a single color or different plural colors, each of the organic electro luminescence elements being provided with an anode, a cathode, a polymeric luminescence layer located between the anode and the cathode, and at least one organic electro luminescence element having a hole injection layer; the method comprising:
forming the hole injection layer by the coating of the ink according to one aspect of the present invention by ink jet process.
According to a further aspect of the present invention, there is provided an organic electro luminescence display device comprising:
a two-dimensional array of pixels containing organic electro luminescence elements capable of emitting a single color or different plural colors, each of the organic electro luminescence elements being provided with an anode, a cathode, a polymeric luminescence layer located between the anode and the cathode, and at least one organic electro luminescence element having a hole injection layer;
wherein the hole injection layer comprises solid matters containing a donor molecule comprising polythiophene and/or a derivative thereof, and an acceptor molecule comprising polystyrenesulfonic acid and/or a derivative thereof; a concentration of sulfate ion in the hole injection layer is limited to 125 ppm or less; and components having a molecular weight of 110,000 or less is limited to 35% or less based on an entire solid matter, the molecular weight being converted as sodium polystyrenesulfonate in an aqueous gel permeation chromatography measured by using UV of 254 nm in wavelength.